In orthopedic medicine, physicians often need to correct certain skeletal injuries or deformities with external fixator devices. These deformities can either be an acute or chronic condition that may be corrected over a short or long period of time. These devices use pins or wires attached to the separate bone segments and an external structural frame to align, or fix, the bone segments in a way to aid in repairing the injury or correcting the deformity. When the device is engaged with a patient, the device applies forces to maintain the bone fragments in a configuration prescribed by a physician. Often the physician must gradually adjust the orientation of the bone segments over time, optimally with the capability to adjust the orientation along six degrees of freedom to ensure the bone segments are placed in the correct anatomic condition.
Devices and methods for treating musculoskeletal deformities are well known in the art. Although these devices vary considerably in design, they typically fall into two broad categories, circular ring and unilateral devices. The circular ring device category is exemplified by Ilizarov-type systems, which have two rings connected by linear struts with a fixed or hinged connection at each end of each strut. A device called a space frame, which has two rings connected by six linear struts having a spherical joint at each end arranged in a hexapod configuration, e.g., a Stewart frame, represents an advancement on the original Ilizarov concept. Other embodiments can be combinations of rings being attached to what would be considered a unilater device. These constructs are typically referred to as hybrid devices.
The Ilizarov device is constructed based on the deformity that needs correcting, that is, for a specific patient and a specific deformity on that patient, hinges and struts are added to address each degree of deformity in a specific case. Ilizarov-type devices are often referred to as serial manipulators in that each adjustment relates to a single degree of deformity. This approach requires the frame to be constructed based on the deformity present, resulting in a fairly straightforward method of use but a potentially complex set of multiple and potentially endless configurations. The space frame is a device that conceptually comes in one configuration even though rings and struts can be of differing sizes. The Stewart frame-type space frame is often referred to as a parallel manipulator in that any given adjustment to any of the six struts will result in a change to all six degrees of freedom. This characteristic makes the Stewart frame type device less intuitive to use. A computer program is often required to direct the user in making the adjustments to correct the deformity.
The unilateral device category has several devices that basically consist of a series of orthogonal planar hinges or spherical joints and, in some cases, sliders that can be locked into a particular orientation. Typically these devices can be used to fix bone segments in a particular orientation but not to gradually, and precisely, drive the orientation, since the joints of the device do not have a direct adjustment device associated with each hinge or slider. Instead, the joints of the device must be loosened and then grossly manipulated on the device as a whole. Like the Ilizarov-type circular ring fixators, some of these devices have the ability to drive certain degrees of freedom, typically compression or distraction and, at times, a single revolution about an axis, although these devices often need to be constructed or mounted in a particular orientation depending on the characterization of the deformity. This requirement complicates their use and also necessitates multiple configurations to address the range of deformities that physicians typically encounter. None of the devices in the prior art allow for precise adjustment in six degrees of freedom while the device is engaged with the patient.
Six-degrees-of-freedom refers to movement relative to three orthogonally-opposed axis plus rotation about each of those three axis. These six degrees of freedom are typically referred to as longitudinal, vertical, lateral, pitch, roll, and yaw.
Also, a space frame or unilateral fixator can be grossly manipulated to position the bone fragments of a deformity in the proper place at the beginning of the a deformity correction process. However, a physician may need to reposition the fixator throughout the deformity correction process, to effectuate a proper correction of the deformity. Typical space frames and unilateral fixators must be disengaged from the patient, that is, the devices need to be loosened while attached to the patient, and grossly adjusted each time the physician needs to adjust the orientation of the fixator during the deformity correction process. One reason for this limitation of existing devices is that the adjustment mechanisms of the devices do not provide sufficient mechanical advantage or range of motion to accomplish the fine adjustment. This inability to finely adjust the fixator in place reduces the utility of these fixators.
What is needed is a device that can address a six-degrees-of-freedom musculoskeletal deformity, that is, a deformity having anterior-posterior (AP), lateral, and axial offsets and angulations or any subset thereof. The desired device should be able to be placed on the deformity in any particular orientation within the limits of practicality and be able to reduce the deformity to the anatomic state without needing multiple configurations of the same device. Additionally, the device should be able to allow for the gross manipulation of the deformity for acute cases and then to allow for additional precise, fine adjustment to be administered during a deformity correction process while the device remains engaged with the patient to fully restore the deformity to the correct anatomic condition through an adjustment mechanism that offers a high degree of mechanical advantage.